This invention relates to electrolytic production of aluminum and more particularly, it relates to an improved anode and/or lining composition for use in a cell for the electrolytic production of aluminum.
In the electrolytic reduction of aluminum, there is great interest in utilizing an anode which is substantially inert to the electrolyte and which does not react with oxygen during cell operation. Anodes of this type are described in U.S. Pat. No. 4,399,008 which discloses a composition suitable for fabricating into an inert electrode for use in the electrolytic production of metal from a metal compound dissolved in a molten salt. The electrode comprises at least two metal oxides combined to provide a combination metal oxide.
Also, U.S. Pat. No. 5,284,562 discloses an oxidation resistant, non-consumable anode for use in the electrolytic reduction of alumina to aluminum, which has a composition comprising copper, nickel and iron. The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode. The electrolyte is preferably composed of a eutectic of AlF3 and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF and LiF.
U.S. Pat. No. 5,069,771 discloses a method of electrowinning a metal by electrolysis of a melt containing a dissolved species of the metal to be won using a non-consumable anode having a metal, alloy or cermet substrate and an operative anode surface which is a protective surface coating of cerium oxyfluoride preserved by maintaining in the melt a suitable concentration of cerium. The anode is provided with an electronically conductive oxygen barrier on the surface of the metal, alloy or cermet substrate. The barrier layer may be a chromium oxide film on a chromium-containing alloy substrate. Preferably the barrier layer carries a ceramic oxide layer, e.g. of stabilized copper oxide which acts as anchorage for the cerium oxyfluoride.
U.S. Pat. No. 3,957,600 discloses anodes of alloys, which may be fragmented and used in baskets, of passive film-forming metals and elements having atomic numbers 23-29 for use in electrowinning metals, methods of using such anodes, and electrowinning cells incorporating such anodes.
Further, U.S. Pat. No. 4,529,494 discloses a monolithic bipolar electrode for the production of primary aluminum by molten salt electrolysis composed of a cermet anodic layer, a conductive and diffusion-resistant intermediate layer, and a refractory hard metal cathodic layer, with the edges covered by an electrolyte-resistant coating. The intermediate conductive layer has a coefficient of thermal expansion intermediate to the anodic and cathodic layers.
U.S. Pat. No. 4,620,905 discloses an electrolytic process comprising evolving oxygen on an anode in a molten salt, the anode comprising an alloy comprising a first metal and a second metal, both metals forming oxides, the oxide of the first metal being more resistant than the second metal to attack by the molten salt, the oxide of the second metal being more resistant than the first metal to the diffusion of oxygen. The electrode may also be formed of CuAlO2 and/or Cu2O.
U.S. Pat. No. 4,871,438 discloses cermet electrode compositions comprising NiOxe2x80x94NiFe2O4xe2x80x94Cuxe2x80x94Ni, and methods for making the same. Addition of nickel metal prior to formation and densification of a base mixture into the cermet allows for an increase in the total amount of copper and nickel that can be contained in the NiOxe2x80x94NiFe2O4 oxide system. Nickel is present in a base mixture weight concentration of from 0.1% to 10%. Copper is present in the alloy phase in a weight concentration of from 10% to 30% of the densified composition.
U.S. Pat. No. 4,999,097 discloses improved electrolytic cells and methods for producing metals by electrolytic reduction of a compound dissolved in a molten electrolyte. In the improved cells and methods, a protective surface layer is formed upon at least one electrode in the electrolytic reduction cell and, optionally, upon the lining of the cell.
U.S. Pat. No. 5,006,209 discloses that finely divided particles of alumina are electrolytically reduced to aluminum in an electrolytic reduction vessel having a plurality of vertically disposed, non-consumable anodes and a plurality of vertically disposed, dimensionally stable cathodes in closely spaced, alternating arrangement with the anodes.
U.S. Pat. No. 4,865,701 discloses that alumina is reduced to molten aluminum in an electrolytic cell containing a molten electrolyte bath composed of halide salts and having a density less than alumina and aluminum and a melting point less than aluminum. The cell comprises a plurality of vertically disposed, spaced-apart, non-consumable, dimensionally stable anodes and cathodes. Alumina particles are dispersed in the bath to form a slurry. Current is passed between the electrodes, and oxygen bubbles form at the anodes, and molten aluminum droplets form at the cathodes. The oxygen bubbles agitate the bath and enhance dissolution of the alumina adjacent the anodes and inhibit the alumina particles from settling at the bottom of the bath. The molten aluminum droplets flow downwardly along the cathodes and accumulate at the bottom of the bath.
Additional anode compositions are described in U.S. Pat. Nos. 3,943,048; 4,049,887; 4,956,068; 4,960,494; 5,637,239; 5,667,649; 5,725,744 and 5,993,637.
There is still a need to improve the corrosivity and conductivity of the non-consumable anode to ensure an anode that provides satisfactory performance without dissolution in an electrolytic cell where alumina is reduced to aluminum.
It is an object of this invention to provide an improved anode for use in an electrolytic cell.
It is another object of this invention to provide an improved composition for an anode having resistance to molten electrolyte salts in an aluminum producing electrolytic cell.
Yet, it is another object of the invention to provide a process for electrolytically producing aluminum from alumina in a low temperature cell using an improved anode.
These and other objects will become apparent from a reading of the specification, claims and drawings appended hereto.
In accordance with these objects, there is provided a method of producing aluminum in an electrolytic cell comprising the steps of providing molten electrolyte in an electrolytic cell, said cell having alumina dissolved in the electrolyte. In addition, anodes and cathodes are provided in the cell, the anodes comprised of Cuxe2x80x94Nixe2x80x94Fe alloys containing about 0.1 to 5 wt. % carbon, incidental elements and impurities. Electric current is passed between an anode and a cathode in the cell and aluminum is formed at the cathode.
The anode has improved resistance to oxidation and corrosion in molten electrolyte baths compared to other anode compositions in the same bath. The anode is comprised of Cuxe2x80x94Nixe2x80x94Fe alloys containing 0.1 to 5 wt. % carbon. Preferably, the anode composition is comprised of 15 to 60 wt. % Ni, 1 to 50 wt. % Fe, 0.1 to 5 wt. % C, the remainder Cu, incidental elements and impurities. A more preferred anode is selected from a composition in the range of 10 to 70 wt. % Cu, 15 to 60 wt. % Ni, 15 to 40 wt. % Fe and 0.1 to 5 wt. % C. A typical composition for the anode would contain 30 to 50 wt. % Cu, 20 to 40 wt. % Ni, 20 to 40 wt. % Fe and 0.7 to 2 wt. % C, with a specific composition containing about 41 wt. % Cu, 28 wt. % Ni, 30 wt. % Fe and 1 wt. % C.
Another feature of the present invention is a cell vessel interior lining which is impervious to penetration by molten electrolyte, which can be readily replaced and which may be readily recycled. The lining covers the bottom and walls of the vessel interior and may be composed of an alloy having substantially the same composition as the anode composition described herein. Located between the external shell and the interior metal lining of the vessel is refractory material, such as alumina or insulating fire brick, which thermally insulates the bottom and walls of the vessel. The interior metal lining may be electrically connected to the anodes, and the walls or bottom or both then constitute part of the anode arrangement. During operation of the cell, oxygen bubbles are generated at the bottom and elsewhere on the interior metal lining when the latter is part of the anode arrangement, and these bubbles help to maintain in suspension in the molten electrolyte the finely divided alumina particles introduced into the cell.
The anodes of the present invention may be fabricated by casting a Cuxe2x80x94Nixe2x80x94Fexe2x80x94C melt of the desired composition. Or, the anodes may be fabricated from sintered metal powders of the desired proportions to produce an anode having a porous surface and a density substantially less than the theoretical density for a given composition (e.g., 60-70% of theoretical density). These anodes have resistance to corrosion by oxidation, when immersed in the cell""s electrolyte. However, the denser anodes have a greater resistance to oxidation in air. The cast anodes have the advantage that they produce a very hard protective coating during use in the cell.
Preferably, a cell in accordance with the present invention employs, as an electrolyte, a eutectic or near-eutectic composition consisting essentially of 42-46 mol. % AlF3 (preferably 43-45 mol. % AlF3) and 54-58 mol. % of either (a) all NaF or (b) primarily NaF with equivalent molar amounts of KF or KF plus LiF replacing some of the NaF.